1. Field of the Invention
This invention relates to a heatable plasticizing assembly for use in an injection molding machine, which assembly comprises a heatable nozzle body, a rotatable feed screw, and a plasticizing cylinder defining a high-pressure chamber and a cylinder low-pressure chamber, which contains said feed screw, which is provided at its forward end with a back flow valve, wherein the high-pressure chamber comprises a cylindrical portion and is continued in the nozzle body by an axially extending portion tapering from said cylindrical portion to the diameter of the nozzle passage, and said cylinder and nozzle body are connected by a plurality of fixing screws disposed on a circular line.
The term "high-pressure chamber" describes a chamber which is disposed in front of the back flow valve and during the injection of molten plastic material into the injection mold is hermetically sealed from the low-pressure chamber by said valve. The high-pressure chamber has the largest volume when the feed screw is in its rearmost position. That rearmost position differs from an actual rear position of the feed screw, which actual rear position is determined by the quantity of plastic material required for a given molding to be made.
2. Description of the Prior Art
A plasticizing assembly of the type described first hereinbefore is known from the periodical "Plastverarbeiter", 41th Year 1990, No. 8, page 114.
In conventional plasticizing assemblies the gap between the nozzle body and the plasticizing cylinder adjoins the high-pressure chamber along the inside peripheral surface of the plasticizing cylinder. That gap is exposed on its inside to the pressure in the high-pressure chamber during the injection of the plastic material and its length is relatively large because it is determined by the inside diameter of the high-pressure chamber. Pressures up to and above 2500 bars may occur in the high-pressure chamber during the injection stroke and will be applied to said gap. In order to ensure a sufficiently tight seal between the nozzle body and the plasticizing cylinder, the contact pressure between the rear end face of the nozzle body and the forward end face of the plasticizing cylinder must be very high. Under that high contact pressure, the screw threads of the relatively long fixing screws are axially forced strongly against the internal screw threads in the tapped bores of the nozzle body and of the plasticizing cylinder.
Prolonged injection molding operations may cause the side faces of the screw threads to stick together owing to the high temperatures to which said screw threads are subjected in the nozzle body and in the plasticizing cylinder. In recent times, it is frequently required to make injection moldings meeting special requirements from materials or mixtures which can be injection-molded only with difficulty. Some of said materials and mixtures tend to release corrosive gases under the extremely high pressure in the high-pressure chamber and said gases enter the gap between the nozzle body and the plasticizing cylinder and reach the screw threads of the fixing screws. At the high temperatures prevailing, such gases may cause the screws to stick more firmly in the tapped bores as a result of corrosion so that it may be difficult to loosen the screws in the tapped bores when this is desired for a replacement of the nozzle body. Such difficulties will be serious because the nozzle body must be replaced rather often for cleaning and repairs and the injection molding operation must often be interrupted for repairs.
U.S. Pat. No. 4,886,439 suggests to connect the nozzle body to the plasticizing cylinder by means of a sleeve, which is adapted to be screw-connected to the plasticizing cylinder and applies an axial pressure to the nozzle body. In that case the nozzle body need not be formed with screw threads, which would give rise to difficulties. On the other hand, the force required to loosen the connecting sleeve is much stronger than the force required to loosen each screw of the plasticizing cylinder described first hereinbefore. Practice has shown that such a connecting sleeve can be loosened only with the aid of a heavy implement consisting, e.g., of a fork spanner, and it is often necessary to strike against the implement in order to loosen said sleeve. Besides, it is often necessary to replace the nozzle body in the premises of the user of the injection molding machine and to have that operation performed by personnel of such user or by service technicians of the manufacturer of the machine. A replacement of the nozzle will give rise to problems particularly when properly skilled labor and proper implements are not available or when it is necessary to transport heavy implements to remote sites of injection molding machines, e.g., in countries which are not highly developed industrially.
In known plasticizing cylinders the effective length of the feed screw usually does not exceed 20 times the diameter of said screw. On the other hand it is often desired to increase the production rate of the injection molding machine by a supply of heat at a higher rate to the plasticized plastic material. In known injection molding machines this can be accomplished only by a supply of a higher electric power to the heating cuffs although the power which can thus be supplied cannot be increased indiscriminately.